Heretofore, as light beam splitting means for splitting a light beam into two beams incident on respective light receiving elements disposed at positions spaced apart from each other, there has been an amplitude division method making use of a semi-transparent mirror.
For varying the ratio between the transmittivity and reflectivity of the semi-transparent mirror which effects the aplitude-based beam splitting, it has been a practice to deposit a metal (such as Al, Ag, Cr or Au) or a non-metal (such as ZnS, CeO, TiO.sub.2) of a suitable thickness.
However, it has been very difficult to control the ratio of the light beam splitting by controlling the film thickness. In addition, light is greatly absorbed, and also the division ratio is subject to great variations with respect to polarization and wavelengths.
The present invention accordingly seeks to solve the above problems through wavefront splitting instead of the amplitude splitting as effected by the prior art semi-transparent mirrors.